adt-run needs a virtual machine. I know that some developers have
some workarounds, but couldn't autopkgtest also support running tests
on the local system? This would be tremendously useful when the tests
can be contained in the binary packages, as it would make it very easy
for our users and ourselves to test the packages.

Thanks to Michael's hint, I found that the following command will work
locally.

I guess that the next step is to let autopkgtest run as a user
(http://bugs.debian.org/648148), make it easier to invoke
(adt-virt-null is not mentionned in the manual page, and even with
--no-built-binaries it still tries to download some stuff or create a
GPG key), and give it a more user-friendly output.